Direct-buried cable is a type of communications or transmissions cable that is manufactured for installation under the ground and in direct contact with the earth without any covering, sheathing, or piping to protect the cable. Direct-buried cable is easier and less expensive to lay under the ground than other cable designs that require protection from the earth and, therefore, are attractive to telecommunications companies. Direct-buried cable includes coaxial and bundled fiber optic cable, and some power cabling. Installations of direct-buried cable typically place cable in trenches at depths of, for instance, from about 30 to 45 inches in order to help to avoid or limit exposure of the cable to corrosion and excavation damage. Direct-buried installations are often combined with cable duct installations.
Without any protective covering or cable duct, direct-buried cable is more susceptible than ducted or sheathed cable to being cut during installation back-fill, digging, or excavation, as well as being damaged due to corrosion. Therefore, means of detecting and identifying the location of trenches and thereby the location of direct-buried cable are necessary to excavate cable for field repair or for removal and replacement.
In addition, means of detecting and tracking cable paths and routes are needed before the installation of new cable or re-routing of existing cable installations. Detection means are also essential for locating existing cable installations and to ensure project plans properly delineate cable tracks to avoid accidents related to undetected utility lines or other hazardous underground structures. Detection means can be useful in configuring and locating cable splice sites and transition points between direct-buried sections and duct/conduit sections of cable. Finally, detection means are helpful and in some cases essential for locating underground structures and underground utility lines, including, for instance, electrical, gas, water, and sewage lines.
Prior art means for detection of direct-buried cable include direct-buried copper clad steel wire including an insulating polyethylene jacket. A well-known copper-clad steel wire includes two conductors referred to in the art as “C-wire,” which is laid directly in trenches proximate to direct-buried cable. The copper-clad steel wire includes a core of steel with a copper jacket and a polyethylene insulating jacket that encases the core. The copper-clad steel wire is relatively rigid due to its steel construction and, therefore, is inflexible. While the rugged construction of a copper-clad steel wire can help to provide some desirable mechanical properties, if the copper jacket is breached due to, for instance, damage from back filling during installation or digging during excavation, signal conductivity is lost and corrosion of the inner steel wire is accelerated. In addition, in these instances, any electrical isolation between the steel wire and the ground is no longer maintained. The copper-clad steel wire is particularly less resistant to damage along its exterior because the steel wire demonstrates an anvil effect due to its higher modulus of elasticity (than copper or aluminum). The steel wire helps to create a cutting force or peak pressure at the site of a force or an impact against its exterior that thereby facilitates a breach of the polyethylene and copper-clad jackets.
In addition, due to the magnetic permeability of the steel core of a bi-metallic wire, high frequency electrical signals migrate to and travel along the copper surface of the steel wire, rather than within the steel core. This creates what is referred to in the art as the “skin effect.” The skin effect increases the wire impedance and reduces the signal applied to the wire. This effect in turn reduces the signal transmitted to a remote device typically used to locate the wire and thereby to locate buried cable or cable routes. In these cases, the wire's conductivity is reduced significantly and its usefulness in locating buried cable is compromised.
Thus, it is highly desirable to have a tracer element for reliable detection of the placement and location of direct-buried cable, as well as placement and location of other utility delivery lines, that is relatively flexible to help to facilitate installation. It is also desirable to have a tracer element that demonstrates strength and resistance to a force or an impact to its exterior to help to minimize or eliminate the tracer element's susceptibility to breach during, for instance, installation, back filling or excavation. Such a tracer element design would demonstrate cut-resistance and would maintain jacket integrity and corrosion resistance comparatively longer than prior art copper-clad steel wire, and, therefore, would have a longer service life. In addition, it is highly desirable to have a tracer element for detecting cable and cable paths that has better conductivity and is less expensive to manufacture than prior art steel wire designs. Finally, it is highly desirable to have a tracer element for use in a wide variety of detection applications for detection and location of underground cable, cable paths and routes, underground structures, and underground utility lines, including electrical, gas, water, and sewage lines.